Frequently Asked Questions

Frequently asked questions about using copper thermite to ignite motors.

General Info

Q: Why use Copper Thermite to light motors?

What is the reason for using copper thermite? Why go to the trouble, when other ignition methods are less hassle?

A:

What is the reason for using copper thermite? Why go to the trouble, when other ignition methods are less hassle?

As a general rule, larger solid motors tend to take longer to fully ignite and come up to pressure than smaller motors.  If you're only lighting a single motor, then a second or two of chuffing on the pad is probably not a big deal.  However, if you're lighting a cluster of motors, a safe flight may depend upon ensuring that all of the motors light simultaneously.

Also, when a motor chuffs and struggles to come fully up to pressure, it is burning propellant that could be put to better use (flying the rocket), and it may also be burning some of the delay charge, resulting in a premature ejection of the parachute.

Copper thermite, properly used, allows you to acheive "instant on" ignition of solid motors.  The thermite lights easily from an electric match, and instantly throws a fine mist of molten copper metal at the walls of the motor core.  The hot metal droplets ignite the entire inside surface of the motor simultaneously, resulting in an "instant on" chuff-free motor ignition.

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Q: How does copper thermite differ from other thermites?

How does copper thermite differ from other thermites? What makes it special?

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How does copper thermite differ from other thermites? What makes it special?

Broadly speaking, a thermite is simply a mixture of a metal oxide and a more-reactive metal.  When ignited, the oxide gives up oxygen, which then combines with the other metal, making a different oxide.

The thermite mixture that most people are familiar with is Iron Oxide (aka "rust") and Aluminum.  This mixture is extremely hard to light, but once it does get going, it produces a slag of aluminum oxide floating atop a blob of molten iron as its reaction products.  Railroads used to use this thermite mixture to weld train tracks together.

Aluminum is a very reactive metal, with a high affinity for oxygen.  In fact, if you saw through a block of aluminum in the open air, a thin coating of aluminum oxide will form over the newly-exposed surface within seconds.

Iron holds on to its oxygen fairly tightly, but not as strongly as aluminum does.  Therefore, while it takes a fair bit of energy input to get iron oxide to let go of its oxygen, the overall reaction is "exothermic" and self-sustaining.

Copper oxide is much easier to break down than iron oxide.  It only takes a little bit of energy input to get it to give up its oxygen, which then combines immediately with the aluminum.

In chemistry terms, we look at balanced equations for the reactions, and evaluate the net change in enthalpy, based on each compound's enthalpy of formation.

 

Enthalpy of Formation values
Compound Heat of Formation
Iron (III) Oxide, Fe2O3 -824 kJ/mol
Copper (II) Oxide, CuO -157 kJ/mol
Aluminum Oxide, Al2O3 -1676 kJ/mol

So, if we scale to multiples of 1 mol quantities:

1 mol (Fe2O3 -824 kJ/mol) + 2 mol Al ---> 1 mol (Al2O3 -1676 kJ/mol) + 2 mol Fe + 852 kJ excess heat

3 mol (CuO -157 kJ/mol) + 2 mol Al ---> 1 mol (Al2O3 - 1676 kJ/mol) + 3 mol Cu + 1205 kJ excess heat

 

It only takes 157 kJ to disassociate each mol of CuO, compared to 824 kJ to dissasociate each mol of Fe2O3.  Therefore, it's much easier to light copper thermite than iron thermite.  In both cases, however, a lot of excess heat is produced, making the reactions exothermic (literally "outputting heat") and self-sustaining.

To reliably light an iron thermite mixture, it takes something like a burning piece of magnesium ribbon, or a fireworks sparkler.  To reliably light a copper thermite mixture, you just need the tiny flame from an electric match.  Copper thermite is MUCH more practical to use for motor ignition.

 

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Q: What is stoichiometry?

What is "stoichiometry" (besides just a cool word to say), and why should I care about it?

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What is "stoichiometry" (besides just a cool word to say), and why should I care about it?

The word "stoichiometry" comes from two Greek words: "stoicheion" (element) and "metron" (measure). It is the scientific method of determining the relative measures of the various elements that take part in a chemical reaction.

If all of the compounds that go into a reaction are in a "stoichiometric ratio", then there is no "extra" of any of the reactants: if they react as much as possible, then there will be none of the original chemicals left over.

This is important for two reasons: First, because we want to avoid wasting chemicals, and second, because the reaction will proceed as quickly and predictably as possible when the reactants are thoroughly pre-mixed in a stoichiometric ratio.

To find the stoichiometric ratio, we first work out a balanced chemical equation, and then work out the mass ratios using the atomic weights of the elements involved.

For instance, looking at the reaction of Hydrogen and Oxygen to form water:

H + O --> H2O

First problem: Hydrogen gas is always found as H2, not as monatomic hydrogen, and the same holds true for oxygen, as well (O2 instead of just O).

We need to fix the reaction formula to reflect that:

H2 + O2 --> H2O

The next problem: Too many Oxygens on the left side. (O2 has 2, and the O in H2O is just 1). So, we double the entire water molecule:

H2 + O2 --> 2H2O

New problem: too much Hydrogen on the right side. 2H2O has 4 hydrogen, so we need to double the hydrogen molecule on the left side:

2H2 + O2 --> 2H2O

This balances: 4 hydrogen atoms + 2 oxygen atoms yields 4 hydrogen atoms + 2 oxygen atoms.

So, we next look up the atomic weights (aka atomic masses) of the atoms involved. Use your periodic table (WebElements.com has a nice .pdf one you can print out). Hydrogen is about 1.0, and Oxygen is about 16.0.

So, for every 2 x (1.0 x 2) = 4.0 parts of Hydrogen, if you add (16.0 x 2) = 32.0 parts of oxygen, you'll end up with 2 x ( ( 1.0 x 2 ) + ( 16.0 ) ) = 36.0 parts of water.

The stoichiometric ratio of hydrogen to oxygen (H:O) by mass is 4:32 (or 1:8, if you simplify by dividing both numbers by 4). It doesn't matter whether you're talking grams, kilograms, pounds, or tons -- any 1:8 mass ratio of hydrogen:oxygen will react completely to form water.

Similar techniques apply to every chemical reaction, including thermite reactions:

3CuO + 2Al --> 3Cu + Al2O3
CuO + Mg --> Cu + MgO

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Safety

Q: Is copper thermite dangerous to use?

Is copper thermite dangerous to use? What precautions do I need to take?

A:

Is copper thermite dangerous to use? What precautions do I need to take?

Any energetic chemical reaction is potentially dangerous.

Copper thermite, by design, produces an expanding cloud of molten copper droplets when it is ignited.  If you have a body part, such as a hand, nearby when it ignites, you WILL be severely burned.  There is simply no way to make an effective motor ignition aid that doesn't dump a lot of heat energy into its surroundings when lit -- doing so is exactly what makes them effective motor ignition aids!

That being said, with a few sensible precautions, it's possible to manage and minimize the risks, and mitigate the potential hazards of using copper thermite. Additionally, the instant ignition afforded by copper thermite can reduce or eliminate the risks associated with off-axis thrust due to partial ignition of motor clusters, or the risks of early ejection caused by burning off part of the motor delay while the rocket sits and chuffs on the launch pad before coming up to pressure and fully igniting.

On balance, using copper thermite properly can be less risky, overall, than not using it.

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Usage

Q: What chemicals should I use?

Which specific chemicals should I use to make copper thermite for motor ignition? Why?

A:

Which specific chemicals should I use to make copper thermite for motor ignition? Why?

The most commonly used (and therefore, the most well-understood and probably the safest) chemicals to make copper thermite for motor ignition are:

  • 325 mesh Aluminum powder (either spherical atomized or flake)
  • Copper (II) Oxide (aka cupric oxide, black copper oxide, or CuO) powder (unknown mesh size, but it's the only one sold by FireFox)

Generally speaking, the finer the chemicals, the more thoroughly and intimately they can be mixed, and the faster the resultant reaction will be.  Faster doesn't always mean better!  If the reaction goes too fast, the mist of copper droplets will be too fine, and they will transfer their heat to the air in the motor core rather than the actual propellant, possibly resulting in nothing more than a loud noise and an unlit motor.  Also, finer mixtures are easier to ignite, and therefore potentially more sensitive or prone to accidental ignition due to static electricity.

We want the reaction to be fast enough to provide near-instant ignition, but slow enough to burn and throw larger copper droplets, rather than pop, when lit, and we want the chemicals to be relatively safe to handle and mix.

A number of rocketeers have experimented with various sizes of aluminum powder in copper thermite mixtures, and 325 mesh seems to provide the optimum balance between slow enough to guarantee motor ignition, and fast enough to make it "instant", without risking accidental premature ignition.

400 mesh Aluminum works in place of 325, although it does react quite a bit faster. If you can't find 325 mesh, 400 mesh is a marginally acceptable substitute. Try to find 325 mesh, however.  It's much more trustworthy....

Experiments with smaller aluminum particles (625 mesh spherical) have been very unsatisfactory: they burn far too fast, resulting in a sharp "crack", rather than a "poof" when ignited.  Because of this, the 625 mesh aluminum hasn't seen any practical use, and its sensitivity (to static, friction, impact, and other potential sources of premature ignition) is unknown, but presumed to be higher than desired.

Therefore, we recommend that you DO NOT EVER USE metal powders finer than 400 mesh in copper thermite mixtures for motor ignition under any circumstances. 325 mesh is optimal.

Coarser metal particles have been tried with some success, however. An aluminum flake powder intended for use in automotive paints, with approximately a 125 mesh size, was tried, and worked successfully, but motor ignition was slightly delayed.  If your goal is "instant on" ignition of your APCP rocket motors, you should use 325 mesh.

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Q: What about other metals?

Can I use any other metal besides aluminum? What about magnesium or magnalium?

A:

Can I use any other metal besides aluminum? What about magnesium or magnalium?

Both magnesium and magnalium (a 50/50 blend of Mg and Al) powders have been tested instead of aluminum, and both worked just fine at 325 mesh.  If you can't get 325 mesh Al powder, substituting 325 mesh Mg powder or 325 mesh Mg/Al powder is acceptable, and performance is nearly identical.

You WILL need to adjust the mixture ratios to achieve a stoichiometric mixture, as the atomic weight of Mg is different than Al, and the reaction product (MgO instead of Al2O3) is different, as well.

Stoichiometric Ratios for copper thermite mixtures with different metals
Metal Mass Ratio (CuO : Metal)
Aluminum 4.422 : 1
Magnesium 3.273 : 1
Magnalium (50/50 Mg/Al) 3.847 : 1

Just to reiterate a point made elsewhere: DO NOT USE METALS FINER THAN 400 MESH. 325 MESH IS OPTIMUM. Whe you use finer metals, the resulting mixtures burn far too fast to be useful as motor igniters, and may be much more sensitive to accidental ignition.

 

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Q: How much should I use?

How much copper thermite should I use to light a particular size of motor?

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How much copper thermite should I use to light a particular size of motor?

How much thermite is needed to light a given motor is NOT a completely "settled" subject; there is still room for experiment and debate.

A general rule of thumb that has been used, successfully, by quite a few rocketeers is to use approximately one gram of copper thermite for every 1000 Newton-seconds of total impulse.  So, for a K motor (between 1280 and 2560 N-s), use between 1.2 and 2.5 grams of copper thermite.  (An Aerotech K550, with about 1500N-s of total impulse, will light instantly with 1.5 to 2.0 grams of copper thermite.)

Experience has proven that the exact total quantity of thermite used is not terribly critical, as long as it's "in the ballpark":  The range of quantities that can work well in a given motor is quite wide -- often 50% more or less will both work fine.

It appears that the reason for this wide range of "successful working quantities" is the fact that the reaction products are all liquids or solids; without any gaseous products, there is very little risk of over-pressurizing the motor core.  As long as you meet a minimum threshold of "hot copper droplets impacting the motor core", you'll light the motor. After that, up to a certain point, more copper thermite just results in more copper being pushed out the nozzle to deposit upon the blast deflector.

(Of course, if you WAY overdo it, you can clog the nozzle with slag, or penetrate too deeply into the surface of the propellant, resulting in a motor cato.  It is generally quite difficult to do this, however -- in many cases, using 4 or 5 times more thermite than necessary to light a motor will NOT result in any problems.)

From a purely physical standpoint, motor core interior surface area and volume are probably the most significant factors determining how much thermite is "enough". However, due to the fact that motor volume scales roughly with total impulse, and interieor surface area of BATES grains scales roughly with the 2/3 power of total impulse, using a rough measure based upon motor total impulse is a reasonable thing to do -- espescially since there is little risk of problems from "over-thermiting" a motor.

Experimentation and data collection to determine the smallest thermite charges necessary to reliably and instantly light particular motors is a very worthwhile avenue for research. More data would be greatly appreciated!

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Q: How should I mix and use copper thermite?

I've decided to use copper thermite to ignite my motor(s). How should I measure and mix it, and how should I construct my "motor ignition devices" using it?

A:

I've decided to use copper thermite to ignite my motor(s). How should I measure and mix it, and how should I construct my "motor ignition devices" using it?

Here's a quick list of the supplies and equipment you ought to have on-hand if you want to work with copper thermite:

  • Nitrile or rubber gloves. Buy inexpensive ones, but wear them.  The chemicals you'll be using aren't particularly poisonous or caustic (in fact, they're fairly benign when separated), but observing proper safety protocols is always a good habit to get into. Plus, fine powders tend to stick to anything they touch, and you don't want to be transfering shiny silver smudges of aluminum powder and black streaks of copper oxide powder to everything you touch for the next few hours.
  • Paper towels.  Either use a dispenser, or tear off some sheets from the roll before you start working, and keep them handy,  That way, you won't have to get the roll dirty when you actually only need one sheet to wipe something off. If you dampen one, it will aid in cleaning up any powder spills.
  • A non-windy place to work, with a solid table to work on.
  • A metal "burn bucket" for disposal of contaminated paper towels and excess thermite mixture.
  • For measurement, an inexpensive 100 gram digital scale with 0.1 gram precision is sufficient for almost all uses, and can be found quite cheaply (less than $10-15, including shipping) on eBay, Amazon.com, and a number of other places online.  It doesn't have to be perfect -- and probably won't be, at these prices -- but you are extremely unlikely to find one that won't work well.
  • Small (1 or 2 ounce) plastic "salsa cups" from the nearest Smart & Final (or other restaurant supply house) make excellent disposable containers for measuring and mixing.
  • Wooden craft sticks (think popsicle sticks without the popsicle) work great for mixing
  • Plastic spoons (use a different spoon in each chemical) are great for scooping chemicals out of their containers.
  • A reclosable container of copper oxide powder (CuO).
  • A reclosable container of 325 mesh aluminum powder.
  • Electric matches.
  • "Containment supplies" (see below).

Once you mix your thermite, you are going to be using it to ignite one or more motors.  To do this properly, ensuring near-instantaneous ignition, you want the copper droplet cloud to expand in all directions within the core of the motor, coating (and igniting) it everywhere at once.  One or more electric matches (depending on the size of the motor and your need for redundancy) will provide the initial flame, but we need to construct something to hold the powder in contact with the match(es) until it's lit, which will also not impede the dispersal of the copper cloud once it has been lit.

For smaller motors and nozzle throats, I like to use the paper wrapper from a drinking straw to contain the thermite.  It's fragile and easily combustible, so it won't impede the spread of the copper droplets.  And, it's narrow, so it will easily fir through the nozzle throat of most J or K motors.  Plus, you can use the straw to transfer the thermite to the wrapper -- it's like it comes with a ready-made perfect-size tool.

For larger motors, plastic wrap works well, as does tissue paper.  In a pinch, a tube rolled from a single layer of wide masking tape, or notebook paper, can work quite well.  Just don't make the "container" too strong: you probably want the burning thermite to spread molten copper in all directions evenly, and not to just spray out in one direction.

It's probably instructive to relate the story of igniting a fairly large motor, to give an idea how it all comes together:

I made the ignition package for a Q motor launched at Black Rock a few years back.  It used three electric matches, with their heads spaced about two inches apart from each other, both for redundancy and to ignite the thermite charge at multiple locations along its length. A sheet of notebook paper was rolled around a wooden dowel as a form, and cut and taped into a cylinder one-layer thick.  The match heads were positioned in the center of the tube, lengthwise, and the bottom end of the tube was securely taped to the wires, sealing that end off.  The wires were taped along their lengths to a 1/8" wooden dowel, to provide support. Once the tube was filled with thermite, the top end was taped shut, and we were ready.

At the pad, once the rocket was vertical, the pad was cleared of all non-essential people, and all on-board electronics were armed.  Then, a spare e-match was connected -- far away from the motor -- to the clip leads to test for accidentally-energized launch equipment. It didn't light, and that gave us the confidence to test again, this time with the thermite package laying on the ground at the far end of its dowel and wires.  After that was also confirmed NOT to light, even when the continuity test button was pressed, it was disconnected from the clips again, and installed in the motor.

The ignition package was inserted about three-quarters of the way up the motor core, and the dowel was taped to the launch pad, to hold it in place.  This position was selected because it seemed like it would give a good spread of copper droplets throughout the core. After hooking up the leads, with NO body parts under the rocket, or in a position to get hit by exhaust, the last of us retreated to the minimum safe distance (1500', IIRC) to watch the countdown and launch.

At the end of the countdown, the LCO pushed the button, and the rocket ignited instantly, with no chuffing or hesitation, and leapt off the pad for a perfect flight.

A long discussion had preceeded the construction of this igniter, with different people arguing for more or less thermite to be used to ensure instant and complete ignition of the 8" diameter Q motor.  The general agreement, such as it was, seemed to be that somewhere between 40 and 120 grams of thermite ought to be the "right amount" to do the job.  However, even after all of the discussions, there were still folks arguing for more or less to be used, and no real consensus had ever emerged.

I mixed up 108.4 grams of thermite (4.42:1 ratio, with 20 grams of aluminum and 88.4 grams of CuO), intending to err towards the upper end of the range discussed, but ended up only using 55 grams, as that was all that fit in the tube I had made. Ignition was instant, so it appears that 55 grams was definitely sufficient.

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